1. Field of the invention
This invention relates to an LED (light-emitting-diode) array, which can be used as a light source in a facsimile apparatus, a handy scanner, and the like.
2. Description of the prior art
In a conventional LED array which has been used as a light source in a contact-type linear image sensor, a plurality of LEDs which emit light beams of wavelengths lying in a certain range are arranged in a line on a slat-shaped substrate. The light beams from the LEDs are converged by a rod lens which is placed at a fixed spacing from the LEDs, so as to illuminate an original. The reflected light beams from the original are received by the contact-type linear image sensor, so that the image information in the original is read by the contact-type linear image sensor.
In the LED array of this type, an LED which emits green light of a wavelength of 555 nm or an LED which emits yellowish-green light of a wavelength of 565 nm is generally used. Although these LEDs are disadvantageous in that their output power is comparatively low, they are capable of emitting light beams which are absorbed by portions colored in red in the original, so that they are applicable to a light source in an image sensor used in a facsimile apparatus or the like.
An LED which is capable of emitting red light of the wavelength of 635 nm has also been developed. This LED is advantageous in that its output power is high, but there is a problem that light from the LED is reflected by red portions in the original. The red light is reflected on the original, the surface of which is colored in white, so that it is impossible for an image sensor to identify these red portions in the original For this reason, the LED is not used in an LED array which serves as a light source in an image sensor used in a facsimile apparatus or the like.
Thus, when a conventional LED array is applied to a light source in an image sensor used in a facsimile apparatus or the like, only one type of LED which emits light that is absorbed by the red portions is used. When the image in the original is constituted by black portions and red portions, and the original is subjected to the reading process with the image sensor using the LED of this type, both the black portions and the red portions are read by the image sensor, even when it is only necessary to read the black portions, and the red portions need not be read. In this case, the unnecessary red portions in the original must be eliminated in advance, which makes the reading process intricate.
When the above-mentioned LEDs for emitting red light, which is reflected by the red portions because of its wavelength, is used in the LED array, the red portions in the original reflect the red light, so that the image sensor is not capable of reading the red portions. Thus, when the LED array is provided with the LEDs for emitting green or yellowishgreen light and the LEDs for emitting red light, the original is irradiated with red light, if desired, as well as green or yellowish-green light, so that application of the facsimile apparatus or the like in which the LED array is used is enlarged, and various requirements of users can be fulfilled.
However, in order to obtain an LED array in which different types of LEDs are used to alternatively emit light beams at wavelengths in the different ranges by changing the direction of current-flow, a number of lead wires are necessary, resulting in an LED array with complicated wiring.